With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a Wireless Local Area Network (WLAN) is a technique for wirelessly accessing the Internet at homes or companies or in specific service providing areas by using portable terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), based on wireless frequency technology.
A lot of standardization tasks are being performed since Institute of Electrical and Electronics Engineering (IEEE) 802, that is, the standardization organization of WLAN technology was established on February, 1980.
Initial WLAN technology supported a speed of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a frequency of 2.4 GHz according to IEEE 802.11, but recently can support a maximum speed of 54 Mbps by using Orthogonal Frequency Division Multiplex (OFDM). In addition, in IEEE 802.11, standardizations for various techniques, such as the improvement of Quality of Service (QoS), Access Point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access vehicular environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, are being put to practical use or being developed.
Furthermore, in order to overcome limitations to the communication speed that was considered as being weakness in the WLAN, there is IEEE 802.11n as a relatively recently established technical standard. An object of IEEE 802.11n is to increase the speed and reliability of a network and to extend the coverage of a wireless network. More particularly, in order to support a High Throughput (HT) having a maximum data processing speed of 540 Mbps or higher, minimize an error in transmission, and optimize the data speed, IEEE 802.11n is based on Multiple Inputs and Multiple Outputs (MIMO) technology using multiple antennas on both sides of a transmitter and a receiver. Furthermore, this standard may use not only a coding scheme for transmitting several redundant copies in order to increase data reliability, but also Orthogonal Frequency Division Multiplex (OFDM) in order to increase the speed.
As the WLAN is actively propagated and applications employing the WLAN are diversified, a need for a new WLAN system for supporting a throughput higher than the data processing speed supported by IEEE 802.11n is recently being on the rise. A Very High Throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems which are recently newly proposed in order to support a data processing speed of 1 Gbps or higher.
In IEEE 802.11 TGac that performs the standardization of the VHT WLAN system, active research is being carried out on a scheme using 8×8 MIMO and a channel bandwidth of 80 MHz or higher in order to provide the throughput of 1 Gbps or higher.
An object of the VHT WLAN system is to support an aggregated throughput of 1 Gbps or higher and to obtain a throughput of at least 500 Mbps or higher in one-to-one communication between terminals. To this end, it is necessary for several STAs to efficiently use a channel at the same. In order for several STAs to efficiently use a channel at the same time, an AP uses a Space Division Multiple Access (SDMA) scheme, such as Multi-user MIMO. That is, several STAs are permitted to perform transmission and reception to and from an AP at the same time.
If several STAs transmit and receive frames to and from an AP at the same time, an interference problem may occur. For example, in case of MU-MIMO downlink (DL) transmission, if a second STA is receiving a frame from an AP while a first STA is receiving a frame from the AP, the transmission of the frame from the AP to the second STA may function as interference with the first STA. As technology for solving the interference problem, beamforming transmission utilizing smart antenna technology may be taken into consideration. In a method of reducing interference in MU-MIMO transmission using beamforming, channel estimation and link adaptation procedures, the transmission of relevant control information, and a frame transmission method through the control information between STAs, that is, the targets of MU-MIMO transmission to an AP need to be taken into consideration.